The white door to the garage opened, and in stepped Katrina Mathis, complaining that her lady parts hurt.

“In fact, everything hurt,” said Mathis, a 43-year-old AmeriCorps manager from the District and budding triathlete who blamed her ailments on her Cannondale bicycle.

Like hundreds of other recreational cyclists in the Washington region, Mathis had found her way to the nondescript garage of Smiley el-Abd, tucked away in a quiet Kensington neighborhood.

Two hours later, Mathis emerged exuberant.

The metal cleats of her bike shoes were moved so that her feet were better positioned over the pedals. A 1-millimeter plastic shim was tucked into her right shoe to better align her right knee with the bike frame. And she got a prescription for a new saddle that would shift her weight from her soft tissue, er, lady parts, onto her “sit bones,” the part of the pelvis designed by nature for sitting.

Cycling has exploded in popularity in the Washington region, with miles of bike lanes unfurling on city streets, one of the most popular bikeshare programs in the country, one of the 10 biggest triathlon clubs in the nation and 16 bike racing clubs in the District alone.

But for some cyclists, there’s a dark undertone beneath all those cheery bicycle bells. Many cyclists complain of aching backs, sore shoulders, numb fingers or worse — all signs that the soft human body and the hard metal bicycle are not working in sync.

And an increasing number — from 20-somethings to septuagenarians and from competitive cyclists to weekend riders — are pedaling their way to Smiley el-Abd’s garage. They come from the District, Virginia, Maryland and beyond — two clients have flown in from the United Arab Emirates.

By day, Smiley is a sales representative for Mitsubishi Electric Cooling and Heating. But in the evenings and on weekends, he turns to his true passion: bike fitting. Most of the bikes he transforms cost at least $1,000 — some much more.

A mechanical engineer, Smiley draws on physics, geometry and an obsession for details to make the machine adapt to the rider, not the other way around.

Carrie Regan discovered Smiley in 2011. Regan, 43, was a middle-of-the-pack triathlete whose Cervélo was causing all kinds of pain.

“I had a lot of frustration with bike fits in the D.C. area,” said Regan, who now lives in Knoxville, Tenn. “You go to a bike shop, and it seemed you get a fit from some guy who did a weekend clinic, and within 10 minutes of leaving the store, the fit just isn’t comfortable.”

While biking on Beach Drive in Rock Creek Park, another cyclist told her about a guy named Smiley who fit bikes nearby. “I Googled around and was able to track him down and sent an e-mail asking, ‘Are you the mythical Smiley of whom they speak who does bike fits off Beach Drive?’ ” she said.

In short order, Regan was standing in Smiley’s garage as they both looked at her feet.

Smiley started with cleat placement, explaining that if cleats are in the wrong spot, everything else is going to be thrown off.

“And I thought, That makes so much sense,” she said. “The bike shop, which shall not be named, never even looked at my cleats. ... Everything he did made so much sense.”

Regan wrote a post about Smiley for the message board of the DC Triathlon Club, and Smiley’s business took off.

Smiley el-Abd, 63, was born Ismail but got his nickname as a perpetually cheery toddler in his native Cairo. His parents, Egyptian diplomats, moved the family to Washington when he was 4. Seven years later, they returned to Egypt, but Smiley came back to the United States to attend college, where he took up soccer. He switched to running as a young adult until knee problems developed.

At 38, he started cycling and quickly fell for the sport, which offered just the right mix of mechanics to challenge his mind and athleticism to engage his body.

He bought a Serotta bike and learned so much about its workings that he began moonlighting as the regional representative for Serotta, which was at the forefront of bike frame design. As part of that job, Smiley taught bike shop owners how to fit customers to Serotta bikes. With the rise of the Internet, the days of the traveling salesman seemed numbered, and Smiley left Serotta in 2004. He began doing fittings out of his garage with a handful of clients, all generated by word of mouth.

“For me, the bike fit is about changing people’s lives,” said Smiley, who logs more than 3,000 miles a year on two wheels, including bike vacations with his wife, Karen, on their tandem bicycle. Their last trip was Paris to Amsterdam, close to 400 miles. “It’s about getting people to really enjoy the bike.”

On average, he sees about two new clients a week, charging $275 for a fitting. Adjustments for the next six months are made free of charge, and a fitting for a second bike is $75.

Smiley can complete a fitting in about 20 minutes, but he takes two hours.

“In a bicycle shop with three, four employees working, ain’t nobody going to talk to you for two hours to sell you a Brooks saddle and change the handlebars — it would be cost-prohibitive,” Smiley said. “That’s one thing I can give that bike shops can’t: time.”

He uses his goniometer to confirm the knee and thigh bone angle on Darren Chesser, 39, of Reston, Va. (Nikki Kahn/The Washington Post)

Inside the garage he has turned into a studio, Smiley lifts each client’s bike onto a trainer and asks him to pedal. Six of his custom-built bicycles hang from hooks on a far wall. Shelves are filled with old bike seats, extra handlebars and assorted parts.

He’s old-school, using a protractor, levels and plumb bobs to diagnose problems and devise solutions, tweaking until he transforms ill-fitting two-wheelers into comfortable speed machines. His concession to the digital age is an iPad, which he uses to video each rider to analyze body position on the bike before and after adjustments.

“He showed me the video, and he starts pulling stuff out and starts tinkering, but he’s also explaining as he goes along — ‘We’re doing this because of this, and we’re doing that because of that,’ ” said Kerwin Speight, 34, an executive producer at NBC4, who was feeling extreme lower back pain and foot pain after 15 minutes on his Scanttante 330 road bike.

Clients get a prescription for new parts, which they can buy anywhere and have Smiley install; he doesn’t sell parts. He follows up with e-mails that ask about the quality of the ride. Sometimes he rides with clients to see it for himself.

“I was getting so many e-mails from Smiley that once my phone went off, and my wife asked what it was, and I said, ‘Oh, it must be Smiley with his thought for the day,’ ” said Mike Meredith, a 55-year-old Baltimore resident.

Meredith came to Smiley in June after two other bike fitters failed to adjust his Bianchi to his 6-foot-3, 240-pound frame.

The Bianchi was such a poor fit that Meredith’s hands were going numb. That was especially alarming since Meredith is a craftsman who makes his living with his hands.

“I was so miserable,” said Meredith, who hadn’t ridden a bike in 25 years but wanted to lose weight and control his diabetes.

He dreamed of riding 50 miles.

“Everybody has this job that you do, and you go to work in the morning, and you go home, and it’s the same every day,” Meredith said, as he talked about the appeal of biking. “This is about setting a physical goal, and you kind of meet yourself out there. That’s where you figure out what you’re about, to a degree.”

The chasm between his dream of biking 50 miles and the reality of the painful Bianchi left Meredith in despair. And thinking about the money and time he’d already spent weighed him down further.

“I’m an idiot at this point,” Meredith said. “The bike’s in the basement. I can’t ride the friggin’ bike.”

After hearing Meredith’s tale, Smiley said he would accept payment only if he was able to get the Bianchi to fit him. “It was a challenge,” Smiley said.

He suggested a longer stem, which connects the handlebars to the steerer tube, a shim for Meredith’s right shoe to stop his foot from rolling outward, a different saddle, new seat post and a new handlebar.

After the parts were installed, Meredith rode the bike out of Smiley’s garage and onto Beach Drive. “And it was wonderful,” he said.

On an afternoon in September, Mike Meredith’s cellphone rang. It was tucked in his bike jersey, but he didn’t pick up because he was intent on finishing his ride.

“Everything feels great,” he reported afterward. “No soreness in the ankles, feet, hips, elbows, arms, hands or shoulders. And Smiley’s the guy who gave that to me.”